Astro Eyepieces id spotting scopes? (1 Viewer)

[giosblue]

Astro Eyepieces on spotting scopes?

Any one who's seen any of my posts will know I'm now using a Baader Hyperion zoom in my Optolyth TBS80ED and to very good effect.
When you look to buy these, they are specced different to regular scope EP's.
I've been looking at some Pentax EP's, they're expensive, but the quality seems to be top notch. ( are they better than the Baaders? ) On the Pentax website they show the magnification of various EP's with different scopes

With a Pentax 80mm scope a 14mm EP is 36x, if I work it out from the info I've got from the forum, for mine it works out at 30x.
To work it out accurately I need to know exact focal length of my scope.
How can I find this out? It's seem all scopes with an 80mm objective lens don't have the same focal length. or do they?
Any advice would be greatly appreciated..

Ron

 

[kkokkolis]

You can measure your focal length F by using an eyepiece of known focal length f and apparent field of view, AFOV.
Place a star at the extreme edge and allow it to pass through the center until it exits from the opposite extreme edge. Count the time needed. Better, repeat a number of times and get the mean time for increased accuracy.
Every 24h the sky moves 360 degrees. That's 15 degrees for 60' or 1 degree every 240". Count the seconds needed for transit and you get your true field of view, TFOV.
AFOV=TFOV x magnification => magnification =AFOV/TFOV. You know now the magnification.
Magnification=F/f => F=magnification x f.
Voila.
Pentax XWs are better than Baader Zoom. Televue Delos are about equal (but Pentax show less pincushion I think). For practical reasons I think a combination of a 24mm 68 degrees eyepiece and a wide field Zoom is the best combination.
I use Explore Scientific 24mm and Baader Zoom. A more luxurious combination would be with Televue Panoptic 24 and Leica Aspheric zoom. It gives 82 degrees at 9mm with great eye relief. Docter 12.5mm and Nagler T4 12mm give the same 82 degrees with great eye relief at a lower magnification. Panoptic has narrow eye relief and Hyperion 24mm great eye relief and the best transmission, better than Pentax 20mm etc. Vixen LVW 22mm is another option with 65 degrees and quite good. If you don't mind small eye relief and pincushion (they are astro eyepieces after all) 82 degrees eyepieces in short focal lentghts are lightweight and impressive, Naglers T6 and Explore Scientific 82. But when I'm not using the Baader for terrestrial I use my 10mm and 6mm Delos or 8mm and 5mm Radians and keep 82 and 100 degrees for astronomy use.